Although historically, gasoline engines operated in ways such that particulates were not formed, gasoline direct injection (GDI) engines can have lean burn conditions and stratified combustion resulting in the generation of particulates. Particulate emissions for direct injection engines are being subject to regulations including the upcoming Euro 5 (2009) and 6 (2014) standards. Existing aftertreatment systems for gasoline engines are not suitable for achieving the proposed particulate matter standard. In contrast to particulates generated by diesel lean burning engines, the particulates generated by gasoline direct injection engines tend to be finer and in lesser quantities. This is due to the different combustion conditions of a diesel engine as compared to a gasoline engine. For example, gasoline engines run at a higher temperature than diesel engines. Also, hydrocarbon components are different in the emissions of gasoline engines as compared to diesel engines.
Emission standards for unburned hydrocarbons, carbon monoxide and nitrogen oxide contaminants continue to become more stringent. In order to meet such standards, catalytic converters containing a three-way conversion (TWC) catalyst are located in the exhaust gas line of internal combustion engines. Such catalysts promote the oxidation by oxygen in the exhaust gas stream of unburned hydrocarbons and carbon monoxide as well as the reduction of nitrogen oxides to nitrogen.
With respect to treatment of diesel exhaust gases and particulates, the prior art generally provides for an oxidation catalyst upstream of a particulate filter. A clean-up catalyst downstream of an oxidation catalyst and a particulate filter combination is provided in U.S. Patent Application Pub. No. 2007/0137187 (Kumar). Suitable clean-up catalysts downstream of the filter include another oxidation catalyst or a three-way conversion (TWC) catalyst located on a substrate carrier such as a flow-through monolith.
Particulate filters used in diesel systems have been coated with, for example, soot burning catalysts that facilitate passive regeneration of soot. In addition, U.S. Pat. No. 7,229,597 (Patchett) provides a catalyzed SCR filter downstream of an oxidation catalyst for simultaneous treatment of nitrogen oxides (NOx), particulate matter, and gaseous hydrocarbons. Further, U.S. Patent Application Pub. No. 2004/0219077 (Voss) discloses a catalyzed filter in communication with a diesel engine exhaust. Placement of catalysts on soot filters, however, leads to gradual loss of effectiveness due to deleterious components of the diesel exhaust stream. A sufficient loading of catalyst is needed to achieve treatment goals, but this should be balanced with the need to provide acceptable back pressure in the system.
There is a need to provide exhaust treatment systems for gasoline direct injection engines that emit particulates.